Refactor exports & Add inject() method

[lukeed]

This is my proposed solution to #1 -- it accepts 1 object at a time and pops off the internal mapping's keys as they are used.

I also took the opportunity to apply a default export, as mentioned in #11. Doing so allowed for a simpler to apply a "state" via inject.

const prompt = require('prompts');
//=> the AsyncFunction

const { prompts, prompt } = require('prompts');
//=> non-breaking imports

const { inject } = require('prompts');
//=> or :: prompt.inject

Lastly, I had to do some variable renaming in order to keep the prompt reference to the function & not the variable placeholder.

Tests were added to accommodate the new exports & to assert the new inject() method.

[terkelg]

Looks good to me! We should add this as one of the key feartures in the readme